A key process In deyelopnnental neurobiology is theimanner by which axons projecting from newly born neurons cHoose paths to reach and Innen/atedistarit target organs. In general, this process Is thought to involve cues that direct axonal outgrowth along specific routes. Hoyvever, only a limited number of guidance cues are known, far less than sufficient to explain the complexity of nnamnriial|an nervous system. This proposal addresses the role of endothelin signaling as a ciritical mechanism that controls the projection of sympathetic axons from the superior cervical gariglia (SGG), New observations described in detail in this proposal lead to a model in which vascular smooth muiscle-derived endothelihs provide guidance cues for sympathetic axons from the SCG to take a specific ^ascular trajectory, along the external Carotid artery. A subset of SCG neurons express the type A endothelin receptor (ETA), and the normal function of this receptor is essential for SCG axons to choose between distinct trajectories and to ultimately innervate their appropriate target tissues. In this application, I propose experiments to address and extend a model in which endothelin signaling, emanating from smooth muscle ofthe external carotid arteries, is a required guidance cuefdrSCG axons. Specific Aim1: To define the genetic requirements for endothelin signaling coitiRonents in SeG axon guidance. This aim addresses the niechanisrn Of endothelin ligand production by smoothmuscle specifically of the external carotid arteries. Specific Aim 2: To define the tissue-specific functions of endothelin signaling components in SGG axon guidance. This aim address the extent to which ETA is intrinsically required by SGG neurons to respond endothelins in order to project along the external carotid arteries. Specific Aim3: To define the molecular basis of defective SCG outgrowth in H0xa3 mutant mice. In this aim, I will evaluate how expression of endothelin signaling;components iis perturbed in Hoxa3 mutant mice, which have a comparable SCG outgrowth deficiency as seen in ETA-deficient embryos